Vehicles having an electric or hybrid drive need rechargeable batteries (traction batteries), which generally have a modular structure, to operate their electrical drive machine. In many applications, such rechargeable batteries are differently also referred to as storage batteries. In order to now supply the electrical drive machine of the electric or hybrid drive with electrical energy from the batteries, a circuit arrangement is interposed between the battery modules and the drive machine.
The rechargeable batteries, usually based on lithium, used in electrically driven vehicles have only a limited service life on account of parasitic chemical processes in their interior. Their capacity is reduced with each charging/discharging cycle until the individual battery cells or the battery modules consisting of such cells have to be replaced owing to a lack of performance and capacity. Therefore, it is important to accurately observe the ageing process of the battery cells or battery modules. Various methods and apparatuses for monitoring the ageing state are known from the prior art.
The scientific article “Smith, A. J. et al., J. Electrochem. Soc. 157, A196 (2010)” describes a method which can be used to infer changes in the ageing state (change in the SOH: State of Health) of lithium ion battery cells from the so-called Coulombic efficiency. However, a corresponding additional power electronic measuring and regulating device is needed to carry out such a method.
The method according to the disclosure provides the advantage that no additional power electronics are required.
In the method according to the disclosure for determining the Coulombic efficiency CE of battery modules of a rechargeable battery, provision is made for the Coulombic efficiency to be determined by means of a circuit arrangement connected to the battery modules. This circuit arrangement has a plurality of current paths each in turn having a series circuit of switching modules and at least one power semiconductor element which can be operated in the linear mode and is intended to regulate the current flowing through the respective current path. One battery module is connected to each of the switching modules, and each of the switching modules is in the form of a switching module for selectively connecting the connected battery module in the respective current path (charging or discharging mode) or for alternatively removing the connected battery module from this respective current path (bypass mode). In this case, in each of the current paths, (i) at least one of the battery modules is selected and is connected in the respective current path by means of the switching modules, while all other battery modules are removed from this current path by means of the switching modules, and (ii) the selected battery module is subjected to at least one discharging process and at least one charging process via the respective current path, the corresponding current being accurately set during charging and discharging of this battery module in this current path by means of the power semiconductor element which is operated in the linear mode, and the corresponding charge quantities Qab, Qzu during charging and discharging or variables proportional to these charge quantities being determined by integrating the current over time. The Coulombic efficiency CE defined as
      C    E    =            Q      ab              Q      zu      can then be determined from the charge quantities Qab, Qzu or variables proportional to the latter. In the simplest case, each of the battery modules consists of an individual battery cell. Alternatively, each of the battery modules consists of a series circuit of a plurality of battery cells.
The circuit arrangement is interposed between the battery modules of the rechargeable battery and a consumer to be supplied by the battery or batteries, each battery module being connected to a switching module of the circuit arrangement. During normal operation, the switching modules are used to select individual battery modules for this voltage supply and to connect them to one another in a current path. Such a circuit arrangement is known as a battery direct converter. The battery direct converter can be or is interposed directly, that is to say without further intermediate elements, between the battery modules, on the one hand, and the electrical consumer to be supplied by the battery modules.
The essence of the disclosure is to control a power semiconductor element in the respective current path of the circuit arrangement in such a manner that said element is at least sometimes in the linear mode and the current through the battery cells of the corresponding battery module is regulated very accurately with the aid of this linearly operated power semiconductor element in accordance with current regulation in the charger. A power semiconductor element which can be operated in this manner is generally present in battery direct converters anyway. Therefore, the very accurate setting of the charging or discharging current, which is needed to determine the Coulombic efficiency CE, can be easily implemented without additional power electronics. Only the control of said power semiconductor element would have to be supplemented in order to carry out the method according to the disclosure. However, such control can manage without power electronic components.
The consumer to be supplied by the battery modules is preferably a multiphase electrical consumer, in particular a multiphase electrical machine. In this case, the battery direct converter is a multiphase direct converter which can be interposed directly between the battery modules of the batteries, on the one hand, and the multiphase electrical consumer to be supplied by the battery modules. In this case, the battery modules can be connected in a number of current paths corresponding to the number of phases.
According to one advantageous development of the disclosure, one of the power semiconductor elements of the switching modules forms the power semiconductor element for regulating the current flowing through the respective current path. In this embodiment, the power semiconductor elements of the switching modules are controlled by means of a control device and are operated in the linear mode in order to set the electrical current during the charging process and the discharging process.
One advantageous embodiment of the disclosure provides for each of the switching modules to have a bridge circuit arrangement with two half-bridges, two power semiconductor elements which act as semiconductor current valves and two freewheeling diodes being connected in each of these half-bridges. Therefore, a bridge circuit arrangement in the form of a full bridge results overall for each switching module. In this case, one of the two semiconductor current valves is (reverse) connected in parallel with one of the two freewheeling diodes for each half-bridge. The two parallel circuits with the one semiconductor current valve and the one freewheeling diode each are connected in a series circuit, thus producing the respective half-bridge. These series circuits of the bridge circuit arrangements are connected to the connected battery module. Such switching modules are known from direct converters, for example, and are used there for so-called “cell balancing”, the equalization of the state of charge between the individual battery cells or battery modules. For this purpose, the battery cells or battery modules are preferably connected, by means of the switching modules, in the current path whose state of charge is relatively high. The main task of the switching modules in direct converters is admittedly to set the voltage at the consumer, that is to say to provide a three-phase voltage system, for example. Strictly speaking, current is generally regulated in the machine in this case and the voltage is set using the switching modules such that the desired nominal current value is produced.
According to another advantageous development of the disclosure, the circuit arrangement is in the form of a circuit arrangement for electrically supplying the multiphase electrical consumer, in particular the multiphase electrical machine. In this case, the number of current paths corresponds to the number of phases. In this case, the electrical supply for each of the phases is effected via a permanently assigned current path in each case. The number of phases or current paths is preferably three or six.
According to one advantageous configuration of the disclosure, the selected battery module is discharged via an external electrical component connected to the circuit arrangement. This component is the electrical consumer, in particular.
According to another advantageous configuration of the disclosure, the selected battery module is discharged via a connectable (load) resistor of the circuit arrangement itself. In this case, provision is made, in particular, for the connectable resistor to be selectively connected or disconnected by means of a controllable contactor.
According to another advantageous development of the disclosure, the selected battery module is charged via a charger connected to the respective current path.
The invention also relates to a corresponding circuit arrangement for determining the Coulombic efficiency of battery modules of a rechargeable battery. This circuit arrangement has a plurality of current paths each in turn having a series circuit of switching modules and at least one power semiconductor element which can be operated in the linear mode and is intended to regulate the current flowing through the respective current path. One battery module is connected to each of the switching modules, and each of the switching modules is in the form of a switching module for selectively connecting the connected battery module in the respective current path (charging or discharging mode) or for alternatively removing the connected battery module from this respective current path (bypass mode). The circuit arrangement is set up to subject the selected battery module to at least one discharging process and at least one charging process via the respective current path, the corresponding current in this current path being able to be accurately set during charging and discharging of this battery module by means of the power semiconductor element which is operated in the linear mode, and the circuit arrangement having means for determining the corresponding charge quantities by integrating the current over time during the charging process and the discharging process. The circuit arrangement also comprises a control device for controlling the respective power semiconductor element in the linear mode in order to set the electrical current during the charging process and the discharging process. In this case, either a central control device is provided or alternatively a separate control device is provided for each individual power semiconductor element. The selected battery module is discharged via an external electrical component or via a connectable load resistor of the circuit arrangement itself. The selected battery module is charged via an external charger connected to the respective current path. Corresponding connections are provided for the external devices. The circuit device is, in particular, a circuit arrangement for carrying out the method mentioned at the outset.